Sequential combustion gas turbines are known to comprise a first burner, wherein a fuel is injected into a compressed air stream to be combusted generating hot gases that are partially expanded in a high pressure turbine.
The hot gases coming from the high pressure turbine, which are still rich in oxygen, are then fed into a reheat burner, which is commonly named as second stage combustion, wherein a further fuel is injected there into to be mixed and combusted in a combustion chamber downstream of the reheat burner; the hot gases generated are then expanded in a low pressure turbine.
The reheat burner of the sequential combustion gas turbine has a duct which is often square, quadrangular or trapezoidal in shape, enclosing static vortex generators typically made of tetraedrical elements connected to the walls in an upstream region of the duct and extending into the duct partially.
Downstream of the vortex generators the reheat burner has a lance made of a straight tubular element placed perpendicularly to the direction of the hot gases flow and provided with a terminal portion that is parallel to the direction of the hot gases flow. The terminal portion usually has more than one nozzle that injects the fuel.
During operation the hot gases flow passes through the turbulence generators, for example vortex generators, flute VG lance, flute lobes lance, by increasing its vortices; afterwards the fuel is injected through the lance such that it mixes with the hot gases flow.
Currently downstream of the lance mixing is basically enhanced by a reduction of the cross sectional area of the burner duct, which reduces the effective diameter to length ratio of the burner. In order to minimise the combustor pressure loss the cross sectional area is increased again towards the end of the mixing zone. Such a reheat burner is disclosed in EP 2 420 730 A2 for example. This cross sectional area increase at the downstream end region of the burner duct is limited by potential separation of the flow from the ducts' walls within the mixing zone. Therefore a conflict between achievable mixing quality and pressure loss exists.
Providing large scale and/or small scale structures along the mixing zone for the purpose of increasing vortices is not the means to encounter the problems due to the risk of recirculation zones and therefore flame holding inside the mixing zone. It is also exacerbated that turbulences, which were created by vortex generators and/or lances decreases constantly inside the mixing zone in flow direction. Therefore mixing does not happen as effective towards the end of the mixing zone as it does close to the injection.
Furthermore, in order to increase the gas turbine efficiency and performances, the temperature of the hot gases circulating through the reheat burner should be increased. Such a temperature increase causes the delicate equilibrium among all the parameters to be missed, such that a reheat burner operating with hot gases having a higher temperature than the design temperature may have flashback, NOx, CO emissions, water consumption and pressure drop problems.
To encounter these constraints partially a reheat burner is proposed, see EP 2 420 730 A2, having a mixing zone with a cross section of diverging side walls in the hot gas flow direction, wherein the diverging side walls define curved surfaces in the hot gas flow direction having a constant radius.
Another proposal for reducing the narrative problems is disclosed in EP 2 420 731 A1 which discloses a reheat burner providing a high speed area with a constant cross section along the mixing zone. Downstream in hot gas flow direction to the high speed area a diffusion area borders with a flared cross section.
It is known that at the downstream end of the mixing zone between the mixing zone and the combustion chamber a step in cross section has the effect of a flame holder.